1. Field of the Invention
This invention relates generally to controllable rectifier circuits, and more particularly it relates to a controllable rectifier circuit which automatically reduces the power to a load after a predetermined period of time has elapsed. The invention has particular applications in which the load is a solenoid coil so that full power is applied during the initial pull-in of the coil but reduces the power to a relatively low level after the need for full power is no longer required.
2. Description of the Prior Art
In U.S. Pat. No. 3,161,759, issued to Gambill et al. on Dec. 15, 1964, there is described a control circuit for varying the output of an electric heater which includes the combination of a silicon controlled rectifier and a capacitor operatively connected in a bridge rectifier.
In U.S. Pat. No. 3,663,943, issued to Nakajima et al. on May 16, 1972, there is described an automatic voltage regulating system for a DC load wherein a capacitor is connected between the gate and cathode of a silicon controlled rectifier to turn on the same when the voltage on the capacitor has reached a predetermined level.
In U.S. Pat. No. 4,118,768, issued to Wilson on Oct. 3, 1978, there is disclosed a rectifier and preregulator circuit using a first silicon controlled rectifier to rectify AC voltage, and the rectified voltage is applied across a capacitor.
In U.S. Pat. No. 4,118,769 issued to Wilson on Oct. 3, 1978, there is shown a full-wave demand controlled pre-regulating supply which includes first and second rectifiers for supplying a rectified current or voltage to a storage capacitor located at the output of the circuit.
In U.S. Pat. No. 4,161,022, issued to Kanazawa et al. on July 10, 1979, there is disclosed an AC to DC power supply utilizing a full-wave bridge rectifier incorporating silicon controlled rectifiers to selectively produce a half-wave or a full-wave rectified signal.
None of the above prior art teaches or suggests a controllable rectifier circuit like that of the present invention which provides a full-wave bridge rectifier circuit with only one of the legs having a silicon controlled rectifier. The silicon controlled rectifier is turned off automatically by a control means after a predetermined time has elapsed so as to convert the bridge rectifier from a full-wave rectifier to a half-wave rectifier.
Generally, it is known that there are many electromagnetically-actuated devices which require more power to actuate the device than is needed to keep the device in the actuated position. Specifically solenoids, relays, contactors, and some motors, among others, can function with a greatly reduced power input once the initial power to close the solenoid, relay, or contactors has actuated the device. Lightly loaded DC motors can operate at either reduced field voltage or reduced armature voltage once the motor has overcome the inertia of the motor and load and the motor has reached operating speed.
This invention includes a full-wave bridge rectifier circuit to apply full power to actuate the device that is associated with it and then, after a predetermined time, reduces the power to the device by changing from a full-wave rectified DC voltage to a half-wave rectified DC voltage. While the invention was developed with a solenoid-actuated valve in mind, the principles can apply equally to other devices.
Although DC solenoids have many advantages over AC solenoids, the use of AC solenoids is widespread because AC voltage is more readily available. The use of a full-wave rectifier in conjunction with the DC solenoid allows the advantages of the DC device to be employed with an AC mains.
The DC solenoid is smaller and lighter than the equivalent AC solenoid for the same stroke and pull force, and it does not require shading coils and laminated iron that are associated with AC solenoids. However, DC solenoids require only a fraction of the actuating power to keep them actuated. The one advantage an AC solenoid has over a DC solenoid is that the power input to the AC solenoid reduces when the solenoid completes its magnetic circuit on actuation. The power consumed by an AC solenoid is limited by its inductance. The inductance is much greater with an actuated plunger than when the plunger is not actuated. Although the DC solenoid draws the same power regardless of the state of the plunger, the other advantages of the DC solenoid are such that many are operated from AC mains using full-wave bridge rectifiers incorporated into the solenoid itself.
There have been a number of methods used heretofore to reduce the power to DC solenoids upon full actuation. One that is commonly used is to have two coils on the solenoid. One coil has a fairly low resistance and is used to initially actuate the solenoid. The other coil has a high resistance and is used to hold the solenoid in the actuated position. A switch, which is operated by the movement of the solenoid plunger, is used to switch the voltage from the low resistance coil to the high resistance coil upon actuation. This requires two coils, a mechanical linkage, switch, and, if operated from the AC mains, a rectifier or rectifiers. The subject invention accomplishes the same result as the two coil solenoid with a switch, in less space and at a much reduced cost. The subject invention allows the solenoid to automatically switch, after a predetermined time, to a reduced power condition where the hold-in power is approximately 30 percent of the actuating power.
When using an automatic switch that switches after a predetermined time, if a short power outage occurs, say in the order of two full cycles of 60 cycle power, the device must be able to reset itself so that full power is applied to the solenoid when the power outage is over. A simple resetting circuit is incorporated in this invention that allows the circuit to reset itself in less time than the solenoid takes to open when power to the device is interrupted. This allows the subject invention to be used without danger of failure when short power outages occur. In addition, if the application requires repeated switching of the solenoid on and off, the frequency at which the solenoid can be operated is not limited by the subject circuit but by the drop out time of the solenoid itself.